The present invention relates to an electrical surge protection device in conjunction with a filter for filtering electromagnetic interference (EMI) or radio frequency interference (RFI). More particularly, the invention relates to a phase balanced surge protection and filtering device which may be used to protect sensitive electronic or electromechanical equipment from electromagnetic pulse EMP, transient surges or EMI wherever differential or common mode protection would be required or where cross line protection is required. Additionally, the invention may be used by electronic equipment manufacturers to comply with various regulatory requirements related to electromagnetic susceptibility and emissions or the like created by such equipment.
The majority of electronic equipment produced presently, and in particular computers, communication systems, military surveillance equipment, stereo and home entertainment equipment, televisions and other appliances include miniaturized components and electrical contacts which according to the materials from which they are made or their mere size are very susceptible to stray electrical energy created by electromagnetic interference or voltage transients occurring on electrical lines. Voltage transients can severely damage or destroy such electronic components or contacts thereby rendering the electronic equipment inoperative, and requiring extensive repair and/or replacement at great cost.
Transient voltages occurring on electrical lines can be induced by lightning which produces extremely large potentials in a very short time. These potentials are transmitted via the electrical line to equipment coupled thereto. In a similar manner, extremely detrimental transients could be induced by electromagnetic energy created by a nuclear electromagnetic pulse EMP wherein intense transient electric and magnetic fields having very short rise times and large frequency spectrums are produced. Other sources of large voltage transients are found to be associated with voltage surges occurring upon the switching off or on of some electronic power equipment as well as ground loop interference caused by varying ground potentials.
Electrical interference in the form of EMI or RFI can be induced into electrical lines from such sources as radio broadcast antennas or other electromagnetic wave generators. Alternatively, another source of interference is found to be generated from equipment coupled to the electrical lines, such as computers, switching power supplies and a variety of other components, which may generate significant interference which is desired to be filtered.
Based upon the known phenomena of transient voltage surges and harmful electrical interference or noise, a variety of filter and surge suppression circuit configurations have been designed to filter EMI or RFI interference or suppress large transient voltages appearing on an electrical line. For example, in U.S. Pat. No. 4,760,485, ZnO surge arresters are utilized to replace existing capacitors in an electrical circuit having capacitive elements with values of up to approximately 100 nF. The ZnO surge arresters behave as capacitors at voltages less than their pick up voltage but would pass into a highly conductive state and limit the voltage after such pick up level has been attained. In this way, providing ZnO surge arresters in conjunction with a low pass filter in the manner of this invention will limit voltage surges, emissions and susceptibility as desired.
In U.S. Pat. No. 4,703,386, there is shown a power receptacle and an associated filter to limit EMI and RFI. The RFI/EMI filter is placed in series with an electrically isolated outlet of the power supply system to thereby protect the electronic equipment coupled to the isolated outlet. The filter includes a series of capacitors coupled to a pair of chokes which are in turn coupled to additional capacitors including a common mode bypass capacitor. The circuit essentially forms a low pass filter utilized to eliminate electromagnetic interference occurring at higher frequencies. The circuit may additionally include surge protection comprising three varistors wherein one varistor is coupled to each output terminal of the circuit for common mode protection.
In the above examples, electromagnetic interference filters may be modified to include some form of surge protection or both. The filter and surge protection circuitry may be incorporated into a device, such as a power supply, in order to provide noise reduction and surge protection. Although known filter circuits and/or surge suppression circuits may be easily incorporated into some electronic devices, many times the incorporation of standard circuit components with leads on a circuit board or the like may present problems due to the leads slowing the protective capabilities of the circuit and the bulk of the circuits or the cost thereof. In many situations, electronic devices are not manufactured with protective circuitry, and the user must provide adequate protection at the point of use of the electronic equipment.
Based upon the foregoing, there was found a need to provide point of use surge suppression or electromagnetic interference filtering which can be easily retrofit into the electronic equipment or placed external to the equipment and positioned between the power cables, data cables or other lines which may couple the equipment to a source of interference or possible voltage surges. In this respect, point of use protection devices have been developed as for example, in U.S. Pat. No. 4,794,485 which shows a voltage surge protector for suppressing transient surges at the location of an electrical outlet. The device mounts on the back side portion of an electrical outlet and includes surge protection components therein such as planar varistors which are interposed between the power source and the device to be coupled thereto.
In another example, as found in U.S. Pat. No. 4,720,760, electrical surge protection is provided by a ZnO non-linear resistor device which is in the form of a circular ZnO disk incorporated into a mains electrical plug. A transient overvoltage at any of the plug pins will be suppressed by break down of a respective one or more of the non-linear resistors so as to conduct the transient to ground. The non-linear resistor device is constructed as a disk of non-linear resistor material having a plurality of discrete first electrodes formed on one surface for cooperation with a second electrode formed on the other surface of the disk. In the configuration, the first electrodes are spaced apart from each other on one surface of the disk by a distance relative to the thickness of the disk so as to make the differential surge current conduction path through the disk to the second electrode thereon. Particularly, the first electrodes must be spaced apart by a distance at least equal to or greater than twice the thickness of the disk. The device is designed to produce a permanent short-circuit through the device between the respective pair of electrodes when a surge occurs across the ZnO varistor material disposed between the electrodes.
In another example, as found in U.S. Pat. No. 4,587,589, a voltage limiting feed-through device is disclosed which includes shunt elements connected between a conductor of the feed-through unit and a conducting wall through which the conductor passes. The shunt elements have first and second contact surfaces having a conducting coating and are formed from a varistor material such as ZnO. The construction provides a symmetrical configuration in order to guarantee an even thermal stress on the individual shunt elements in the event of a transient surge voltage. The voltage limiting feed-though units of this invention are suitable for use with conductors which serve to transmit relatively low frequency signals. This construction may be imbalanced with respect to the electrodes due to differences in the thicknesses or the dielectric constants of the shunt elements.
In all, although some point of use protective devices have been developed, a problem exists with enabling both electromagnetic interference and transient voltages to be effectively protected against using an inexpensive device which is both easily manufactured and handled. Additionally, transient overvoltage protection circuitry necessarily must be balanced between electrical lines in a multi-line system to preclude the possibility that a high voltage will be generated due to an unbalanced voltage potential across the lines in the circuit. Another problem with prior art constructions is that each electrical component in the circuit includes leads, wherein the lead lengths inherently will induce back EMF into the circuit and slow the clamping or filtering characteristics of the device. This problem would be increased by balancing the electrodes which would require additional leads.
It has therefore been found that a common electrode between possible elements to create such a balance is desirable. In this regard, there has been developed a three electrode surge arrester formed from a three terminal gas filled tube arrester. The gaps of the individual electrodes are within a single gas filled envelope and therefore tightly coupled such that breakdown of one gap leads to virtually simultaneous breakdown of the other two gaps within the gas filled tube. This gas filled tube arrester has been developed by Harmon Electronics, but has several deficiencies in that the circuit is relatively slow to recover and is bulky with labor intensive manufacture. Although the three electrode configuration creates the balance needed between lines in a system, these circuits do not have EMI suppression capabilities. Additionally, the construction is based upon reducing capacitance in the circuit which is contrary to suppressing EMI by maximizing capacitance.
It has also been found that conventional protective circuits which have attempted to provide surge protection and filtering normally would require more MOV material for higher overvoltages which adversely effects the capacitance value of the components. Generally, the prior art conditioning filters and pulse limiting circuits won't have sufficiently high capacitance as single sheets of material. Multilayer stacks of material which are monolithically fired to form a unit are known and can be used to form multilayer ceramic capacitors for example. Thus, to obtain the desired capacitance, the voltage potential across the component is reduced and the arrangement cannot be used is some high voltage applications. There is thus a need to provide a conditioning circuit arrangement wherein the desired capacitance values may be maintained for the filtering while increasing the voltages which will be effectively clamped by a surge protection device.